The present disclosure relates to electrolysis, particularly to water electrolysis, and more particularly, to increasing corrosion resistance of stainless steel anodes for use in alkaline water electrolysis.
During alkaline water electrolysis for the production of hydrogen, an electrical current is applied between a pair of inert electrodes immersed in an aqueous liquid electrolyte. The liquid electrolyte is frequently a 25 to 30 weight percent (wt %) potassium hydroxide solution. The negatively charged electrode is called the cathode, and the positively charged electrode is known as the anode. Each electrode attracts ions which are of the opposite charge. Therefore, positively charged ions (i.e., cations) move towards the cathode, while negatively charged ions (i.e., anions) move toward the anode. The energy required to separate the ions is in the form of electrical current provided by an electrical power supply that causes the ions to gather at the respective electrodes.
In this process, hydrogen and oxygen are evolved at the cathode and anode, respectively. At the cathode surface, in addition to reduction of protons to hydrogen, positively charged metal ions are reduced to a solid (electrons are gained), thereby creating electrochemical protection from the environment to which it is exposed. However, because the anode has a lower electrical potential (an “under” potential), the anode metal is oxidized (electrons are lost) and experiences a sacrificial weight loss during the electrochemical reaction. This is largely dependent on the specific electrode material, on the operating temperature, and on the amount of electrical current passing through electrode surface (current density). Electricity utilizes ionic species (electrolytes) in the water to travel, therefore the higher the concentration of ions in solution, the further and quicker electricity can travel through the water medium to attack the anode.
Anodes have been formed from nickel or stainless steel. Stainless steels are high-alloy steels that have superior corrosion resistance compared to other steel alloys because they contain large amounts of chromium. Generally, nickel based steels have the greatest corrosion stability in caustic environments when used as an anode in comparison with stainless steels using other metal alloys, especially when exposed to hot caustic. Generally, higher concentrations of nickel provide greater corrosion resistance and stability. However, stainless steels having higher concentrations of nickel are relatively more expensive than stainless steels having lower concentrations of nickel and are also more difficult to machine. Consequently, the use of stainless steel alloys having high nickel contents, or for that matter, the use of pure nickel as an anode material is not considered to be economical in alkaline water electrolysers. Lower concentrations of nickel-based stainless steels are even less preferred due to their lower corrosion resistance and sacrificial loss during use.
Accordingly, there is a need in the art for new materials for use as anodes that can provide increased corrosion resistance to the anode for use in alkaline water electrolysers, wherein the anode is substantially non-reactive and consumption is minimized and/or prevented.